The present invention is related to door control apparatus for passenger carrying vehicles and particularly to such apparatus employing pneumatic door actuators and two-way control valves for supplying and releasing pneumatic pressure at the opposite sides of the pneumatic actuator piston.
Typically, these two-way control valves are solenoid operated, one valve supplying pressure to one side of the actuator piston, while another valve exhausts pressure from the opposite side of the actuator piston. Consequently, one or the other of these solenoid operated control valves is energized while the door is open, and the other is energized while the door is closed. Because the door is either open or closed, it should be apparent that the solenoid windings must operate for an extended duration and therefore must be designed for a 100% duty factor, which results in high costs. In addition, the passenger safety factor dictates that the control valves be capable of exhausting both sides of the pneumatic actuator piston simultaneously in order to allow the door to be manually operated, in the event a passenger or other obstacle becomes lodged in the doorway.
It is known in the prior art to actuate a solenoid valve by a momentary electrical impulse and to maintain the valve in the actuated position by a permanent magnet.